This invention relates to a control valve for a fluid pressure parking brake system.
Many modern heavy duty vehicles equipped with air brakes are commonly also equipped with spring applied, fluid pressure released parking brakes, in which hold-off pressure normally supplied to the spring brakes maintains them in a released condition. However, when this hold-off pressure is vented, the spring actuator automatically effects a mechanical brake actuation. Communication of this hold-off pressure to the spring brakes is normally controlled by a manually operated parking brake control valve. Although the valve is normally manually operated, the prior art valves include a safety feature, so that when the supply pressure drops to a predetermined low value, the parking brake control valve automatically exhausts the spring brake hold-off pressure, to thereby automatically effect a mechanical brake application. However, prior art systems of this type suffer a disadvantage inasmuch as the hold-off pressure required to keep the spring brakes from being gradually applied or "dragging" is much higher than predetermined low pressure at which it is no longer safe to operate the vehicle and at which the parking brake control valve automatically vents the spring brakes. Obviously, this slow application or "dragging" of the spring brakes is undesirable, since dragging brakes increase the wear rate thereof, and often result in a dangerous vehicle operating condition. For this reason, it is desirable to prevent the slow application or "dragging" of the spring brakes, while retaining the safety feature of an automatic spring brake application when the fluid pressure level in the system drops to below a predetermined pressure level at which it is no longer safe to operate the vehicle.